A process for synthesising silver nanoparticles

ABSTRACT

An improved process for synthesising discrete high definition silver nanoparticles in large batch volumes. The method enables the reproducible production of silver nanoparticles having a predetermined size, shape and surface chemistry. The process comprises the steps of forming silver seeds from a reagent comprising, a silver source and a reducing agent; and growing the thus formed silver seeds into silver nanoparticles wherein the step forming silver seeds and/or growing the silver seeds into silver nanoparticles is performed using micro fluidic flow chemistry.

This invention relates to a process for the production of nanoparticles. In particular the invention relates to a process for the production of large quantities of nanoparticles.

Nanoparticles can be synthesised from a range of materials including dielectric inorganic, organic, polymer and metallic materials. Nanoparticles have been utilised in a number of different fields of technology ranging from paints to biomolecular diagnostics. Over the last few years there has been an increase in the number of uses of nanoparticles, such an increase has resulted in a need to producing nanoparticles in large quantities while maintaining batch reproducibility.

WO 04/086044, the entire contents of which is incorporated herein by reference, describes a two-step wet chemistry batch process for synthesising silver seeds and growing the synthesized silver seeds to produce a range of silver nanoparticles. However, the quantity of silver nanoparticles produced by a wet chemistry batch reaction process are limited.

STATEMENTS OF INVENTION

The invention provides a process for synthesising silver nanoparticles comprising the steps of:

-   -   (a) forming silver seeds from a silver source and a reducing         agent; and     -   (b) growing the thus formed silver seeds into silver         nanoparticles         wherein step (a) or (b) is performed using microfluidic flow         chemistry.

Silver nanoparticles produced by the process may have an average diameter of between 5 nm and 100 nm and a UV-vis spectrum peak in the 420 nm to 900 nm region

In one embodiment, both steps (a) and (b) may be performed using microfluidic flow chemistry.

The silver source may be a silver salt, for example silver nitrate.

The silver source may be a complexed silver compound or salt.

The silver source may be dissolved in a capping agent solution, for example a capping agent solution selected from the group consisting: Trisodium Citrate, Cetyl -trimethyl-ammonium-bromide.

The reducing agent may be selected from the group consisting: sodium borohydride, ascorbic acid.

The ratio of silver source: reducing agent may be about 1:8.

Step (a) may be performed using microfluidic flow chemistry with a flow rate of between 3 ml/min and 10 ml/min for the silver source.

Step (a) may be performed using microfluidic flow chemistry with a flow rate of about 1 ml/min for the reducing agent.

Step (a) may be performed at 0° C.

In one embodiment, step (b) may further comprise the step of aging the silver seeds. The aging step may comprise:

-   -   mixing a silver source with a polymeric stabiliser; and     -   mixing the thus formed silver source-polymeric stabiliser         component with silver seeds produced by step (a).

The silver source of the aging step may be the same as the silver source used in step (a).

The polymeric stabiliser may be water soluble. The polymeric stabiliser may have a molecular weight between 10 kDa and 1300 kDa. The polymeric stabiliser may be selected from one or more of the group consisting: poly(vinyl alcohol), poly(vinyl pyrollidone), poly(ethylene glycol), and poly(acrylic acid). For example, the polymeric stabiliser may be poly(vinyl alcohol).

The aging step may further comprise the step of reducing the silver source present in the silver source-polymeric stabiliser-silver seed mixture. The silver source may be reduced by ascorbic acid.

Step (b) of the process may be carried out at a temperature of between 10° C. to 60° C. For example, step (b) is carried out at a temperature of 40° C.

The nanoparticles produced by the process may be stable in an aqueous solution.

The nanoparticles produced by the process may have a colour tunability throughout the visible and near infra red spectrum.

The nanoparticles produced by the process may be red in colour in a colloidal aqueous solution.

The nanoparticles produced by the process may comprise at least 30% non-spherical shaped nanoparticles.

The nanoparticles produced by the process may comprise at least 50% non-spherical shaped nanoparticles. For example, at least 70% non-spherical shaped nanoparticles.

The non-spherical shaped nanoparticles may be triangular and/or hexagonal and/or truncated triangular in shape.

The nanoparticles produced by the process may have a UV-vis spectral peak in the 345 nm region.

The nanoparticles produced by the process may have a UV-vis main spectral width FWHM of less than 300 nm. For example, a UV-vis main spectral width FWHM of less than 150 nm, such as a UV-vis main spectral width FWHM of less than 120 nm or a UV-vis main spectral width FWHM of less than 100 nm.

The invention further provides a process for synthesising silver nanoparticles comprising the steps of:

-   -   (a) forming silver seeds from a silver source solution and a         reducing agent solution; and     -   (b) growing the thus formed silver seeds into silver         nanoparticles         wherein at least step (b) is performed using pressurised         microfluidic flow chemistry.

The silver seeds may be grown in step (b) by mixing a silver source solution and a reducing agent solution. The silver source solution may comprise silver seeds. The reducing agent solution may comprise silver seeds. The silver source solution may comprise a stabiliser. The reducing agent solution may comprise a stabiliser.

The solutions may be pressurised to at least about 35 MPa such as in the range of between about 35 MPa to about 275 MPa. The solutions may be pressurised at about 140 MPa. The solutions may have a shear rate of at least about 1×10⁶ s⁻¹ such as in the range of about 1×10⁶ s⁻¹ to about 50×10⁶ s⁻¹.

The solutions may have a flow rate of at least about 10 ml/min such as at least about 100 ml/min or at least about 1 l/min, for example at least about 10 l/min.

The solutions may be introduced separately. Each solution may have a different flow rate or each solution may have the same flow rate. The solutions may be added at different concentrations.

The residence time of the solutions in a mixing chamber of a microfluidic system may be less than about 1 ms. The residence time of the solutions in a mixing chamber of a microfluidic system may be between about 2 μs and about 1 ms.

Step (b) may be carried out at a temperature of between about 10° C. to about 60° C., for example about 40° C.

The silver source may be a silver salt such as silver nitrate. The silver source may be present at a concentration between about 10⁻³M to about 10⁻¹M.

The stabiliser may be a polymeric stabiliser. The polymeric stabiliser may be water soluble. The polymeric stabiliser may have a molecular weight of between about 10 kDa and about 1300 kDa. The polymeric stabiliser may be selected from one or more of the group consisting: poly(vinyl alcohol), poly(vinyl pyrollidone), poly(ethylene glycol), poly(sodium styrenesulphonate) and poly(acrylic acid). The polymeric stabiliser may be poly(vinyl alcohol). The polymeric stabiliser may be present at a concentration of about 10⁻² wt % to about 10 wt %.

The stabiliser may be trisodium citrate. The trisodium citrate may be present at a concentration of between about 10⁻³M to about 10⁻¹M.

The reducing agent may be ascorbic acid. The reducing agent may be present at a concentration of about 10⁻³M to about 10⁻¹M.

The silver seeds may be present at a concentration of between about 10⁻⁸M to about 10⁻⁴M of silver.

The silver source in step (a) may be a silver salt such as silver nitrate. The silver source in step (a) may be dissolved in a capping agent solution. The capping agent solution may comprise trisodium citrate and/or cetyl-trimethyl-ammonium-bromide.

The reducing agent in step (a) may be sodium borohydride and/or ascorbic acid

The ratio of silver source: reducing agent in step (a) may be 1:8.

Step (a) may be performed using microfluidic flow chemistry. The solutions may be introduced separately. Each solution may have a different flow rate. Each solution may have the same flow rate. The solutions may be added at different concentrations. The flow rate of the silver source solution may be between about 3 ml/min and about 10 ml/min such as about 8 ml/min. The flow rate of the reducing agent solution may be between about 0.5 ml/min and about 1.5 ml/min such as about 1 ml/min.

Step (a) may be performed using pressurised microfluidic flow chemistry. The solutions may be pressurised to at least about 35 MPa. The solutions may be pressurised in the range of about 35 MPa to about 275 MPa such as about 140 MPa. The solutions may have a shear rate of at least about 1×10⁶ s⁻¹ such as in the range of about 1×10⁶ s⁻¹ to about 50×10⁶ s⁻¹. The solutions may have a flow rate of at least about 10 ml/min. The solutions may have a flow rate of at least about 100 ml/min. The solutions may have a flow rate of at least about 1 l/min. The solutions may have a flow rate of at least about 10 l/min. The residence time of the solutions in a mixing chamber of a microfluidic system may be less than about 1 ms. The residence time of the solutions in a mixing chamber of a microfluidic system may be between about 2 μs and about 1 ms.

Step (a) may be performed at about 0° C.

The invention also provides a process for producing silver nanoparticles comprising the steps of:

-   -   (a) preparing silver seeds in the presence of a water soluble         polymer; and     -   (b) growing the silver seeds to form nanoparticles.

The polymer may be a polyanionic polymer. The polymer may be a derivative of polysulphonate. The polymer may be a derivative of polystyrene sulphonate. The derivative may be an inorganic salt of polystyrene sulphonate. The derivative may be a monovalent salt of polystyrene sulphonate. The polymer may be poly (sodium styrenesulphonate) (PSSS). The PSSS may have a molecular weight between about 3 KDa to about 1,000 KDa such as about 1,000 KDa. The concentration of polymer in the silver seed preparation may be about 2.5 mg/ml to about 250 mg/ml such as about 25 mg/ml. The step of preparing silver seeds may be carried out at room temperature. The step of growing the silver seeds may be carried out at room temperature.

The invention further provides a process for synthesising silver nanoparticles comprising the steps of:

-   -   (a) mixing a silver source solution and a reducing agent         solution to form silver seeds; and     -   (b) growing the thus formed silver seeds into silver         nanoparticles         wherein step (a) or (b) is performed using microfluidic flow         chemistry. Step (a) or (b) may be performed in the presence of a         water soluble polymer.

The water soluble polymer may be a polyanionic polymer. The polymer may be a derivative of polysulphonate. The polymer may be a derivative of polystyrene sulphonate. The derivative may be an inorganic salt of polystyrene sulphonate.

The derivative may be a monovalent salt of polystyrene sulphonate. The polymer may be poly (sodium styrenesulphonate) (PSSS). The PSSS may have a molecular weight of between about 3 KDa to about 1,000 KDa such as about 1,000 KDa. The concentration of polymer may be between about 10⁻¹ mg/l to about 250 g/l. The concentration of polymer may be between about 10⁻¹ mg/l to about 1 g/l. The concentration of polymer may be between about 2.5 g/l to about 250 g/l.

Step (a) is performed in the presence of the water soluble polymer.

The silver seeds may be grown in step (b) by mixing a silver source solution and a reducing agent solution. The silver source solution may comprise silver seeds. The reducing agent solution may comprise silver seeds. The silver source solution may comprise a stabiliser. The reducing agent solution may comprise a stabiliser.

The silver source may be a silver salt such as silver nitrate. The silver source may be present at a concentration between about 10⁻³M to about 10⁻¹M.

The stabiliser may be a polymeric stabiliser. The polymeric stabiliser may be water soluble. The polymeric stabiliser may have a molecular weight between about 10 kDa and about 1300 kDa. The polymeric stabiliser may be selected from one or more of the group consisting: poly(vinyl alcohol), poly(vinyl pyrollidone), poly(ethylene glycol), poly(sodium styrenesulphonate) and poly(acrylic acid). The polymeric stabiliser may be poly(vinyl alcohol). The polymeric stabiliser may be present at a concentration of between about 10⁻² wt % to about 10 wt %.

The stabiliser may be trisodium citrate. The trisodium citrate may be present at a concentration of between about 10⁻³M to about 10⁻¹M.

The reducing agent may be ascorbic acid. The reducing agent may be present at a concentration of between about 10⁻³M to about 10⁻¹M.

The silver seeds may be present at a concentration of between about 10⁻⁸M to about 10⁻⁴M of silver.

Step (b) may be carried out at a temperature of between about 10° C. to about 60° C. such as about 40° C.

Step (b) may be preformed using microfluidics. The solutions may be introduced separately. Each solution may have a different flow rate. Each solution may have the same flow rate. The solutions may be added at different concentrations

Step (b) may be performed using pressurised microfluidic flow chemistry. The solutions may be pressurised to at least about 35 MPa. The solutions may be pressurised in the range of about 35 MPa to about 275 MPa such as at about 140 MPa. The solutions may have a shear rate of at least about 1×10⁶ s⁻¹ such as in the range of between about 1×10⁶ s⁻¹ to about 50×10⁶ s⁻¹.

The solutions may have a flow rate of at least about 10 ml/min. The solutions may have a flow rate of at least about 100 ml/min. The solutions may have a flow rate of at least about 1 l/min. The solutions may have a flow rate of at least about 10 l/min.

The residence time of the solutions in a mixing chamber of a microfluidic system may be less than about 1 ms. The residence time of the solutions in a mixing chamber of a microfluidic system may be between about 2 μs and about 1 ms.

The silver source of step (a) may be a silver salt such as silver nitrate. The silver source of step (a) may be dissolved in a capping agent solution. The capping agent solution may comprise trisodium citrate and/or cetyl-trimethyl-ammonium-bromide.

The reducing agent of step (a) may comprise sodium borohydride and/or ascorbic acid.

The ratio of silver source: reducing agent in step (a) may be 1:8.

Step (a) may be carried out at a temperature of about 0° C.

Step (a) may be performed using microfluidic flow chemistry. The solutions may be introduced separately. Each solution may have a different flow rate. Each solution may have the same flow rate. The solutions may be added at different concentrations. The flow rate of the silver source solution may be between about 3 ml/min and about 10 ml/min such as about 8 ml/min. The flow rate of the reducing agent solution may be between about 0.5 ml/min and about 1.5 ml/min such as about 1 ml/min.

Step (a) may be performed using pressurised microfluidic chemistry. The solutions may be pressurised to at least about 35 MPa. The solutions may be pressurised in the range of about 35 MPa to about 275 MPa such as about 140 MPa. The solutions may have a shear rate of at least about 1×10⁶ s⁻¹ such as in the range of about 1×10⁶ s⁻¹ to about 50×10⁶ s⁻¹.

The solutions may have a flow rate of at least about 10 ml/min. The solutions may have a flow rate of at least about 100 ml/min. The solutions may have a flow rate of at least about 1 l/min. The solutions may have a flow rate of at least about 10 l/min.

The residence time of the solutions in a mixing chamber of a microfluidic system may be less than about 1 ms. The residence time of the solutions in a mixing chamber of a microfluidic system may be between about 2 μs and about 1 ms.

The silver nanoparticles synthesised by the processes described herein may have an average diameter of between 5 nm and 100 nm and a UV-vis spectrum peak in the 420 nm to 900 nm region

The silver nanoparticles synthesised by the processes described herein may have an average diameter of between about 5 nm and about 100 nm and an optical absorption spectrum peak in the region of about 900 nm to about 1610 nm.

The nanoparticles may be stable in an aqueous solution.

The nanoparticles may have a colour tunability throughout the visible and near infra red spectrum.

The nanoparticles may be red in colour in a colloidal aqueous solution.

The nanoparticles may comprise at least 30% non-spherical shaped nanoparticles.

The nanoparticles may comprise at least 50% non-spherical shaped nanoparticles.

The nanoparticles may comprise at least 70% non-spherical shaped nanoparticles.

The nanoparticles may comprise at least 95% non-spherical shaped nanoparticles.

The non-spherical shaped nanoparticles may be triangular and/or hexagonal and/or truncated triangular in shape. The non-spherical shaped nanoparticles may be triangular in shape (nanoprisms).

The non-spherical shaped nanoparticles may be plate like having an aspect ratio of between about 1:2 to about 1:10. for example triangle edge length of about 45 nm and height about 5 nm.

The nanoparticles may have an optical absorption spectrum peak in the region of about 340 nm±10 nm.

The nanoparticles may have an optical absorption spectrum peak in the region of about 335 nm to about 338 nm.

The nanoparticles may have a UV-vis spectral peak in the 345 nm region.

The nanoparticles may have a main optical absorption spectrum peak width (FWHM) of less than about 300 nm.

The nanoparticles may have a main optical absorption spectrum peak width (FWHM) of less than about 150 nm.

The nanoparticles may have a main optical absorption spectrum peak width (FWHM) of less than about 120 nm.

The nanoparticles may have a main optical absorption spectrum peak width (FWHM) of less than about 100 nm.

Continuous microfluidic flow synthesis of gold nanoparticles with online optical monitoring has been described however the volume productions are restricted to the order of milliliters per hour. Microfluidic processes of the invention can produce large volumes of high definition silver nanoparticles. The silver nanoparticles produced by a microfluidics process as described herein have improved physical properties, for example narrower size distribution, increased presence of shaped nanoparticles, higher uniformity of samples, better and higher batch to batch reproducibility, compared to nanoparticles produced by a conventional wet chemistry method including,

Silver nanoparticles produced in accordance with the processes described herein may possess one or more of the following characteristics:

-   -   Approximately 30 nm in diameter     -   Narrow distribution of size about the 30 nm median     -   Presence of shaped nanoparticles e.g. triangles, hexagons,         truncated triangles     -   Red in colour when in aqueous solution     -   UV-Vis Spectrum with main peak in the 420-950 nm region     -   FWHM of main UV-Vis spectral peak to be about 150 nm or less     -   Stable in aqueous solution

Improvements in the production of silver nanoparticles using the Microfluidics technology described herein compared to the conventional wet chemistry process include:

-   -   High percentage of triangles vs spheres     -   UV-Vis main spectral peak FWHM of less than about 100 nm

Properties of high definition silver nanoparticles include:

-   -   Size of about 130 nm or less     -   Narrow size distribution     -   Presence of shaped non spherical nanoparticles (for example         triangles)     -   Non aggregated nanoparticles (discrete nanoparticles)

Properties of high definition silver nanoparticles which can be observed in the UV-Vis absorption include:

-   -   Spectral peak in the 345 nm region—this is a key characteristic         of the presence of shaped non-spherical silver nanoparticles     -   Peak or peaks beyond about 420 nm     -   Spectral width of less than about 300 nm or ideally less than         about 150 nm at FWHM

Microfluidics can be used to produce shaped silver nanoparticles with the advantage that the microfluidics synthesis process produced a half litre per batch (and is capable of producing several litres per hour at flow rates typically in the range 10 ml/minute to 500 ml/minute) while the wet chemistry method is limited to 100 ml production. TEM images of microfluidics process produced silver nanoparticles confirmed a significant improvement in the size distribution of the nanoparticles compared to nanoparticles prepared using a conventional wet chemistry technique. We have found that nanoparticles produced by the microfluidic processes described herein have a greater batch to batch reproducibility of physical characteristics compared to nanoparticles produced by a conventional wet chemistry process. Furthermore, nanoparticles produced by the microfluidic process have long term stability in water or aqueous solution suspension, in particular the nanoparticles do not aggregate or sediment over time.

Optimisation of the microfluidic processes, both the chip and the processor routes, will enable a controlled scale-up of the production of high quality high definition silver nanoparticles in a range of shapes, sizes, colours, and/or surface chemistries. The microfluidic processes described herein can be adapted for the scaled up production of a range of high quality nanoparticles.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which:—

FIG. 1 is an example of a microfluidic flow chemistry synthesis for silver seed synthesis (step (a)) of the process for the production of discrete high definition silver nanoparticles;

FIG. 2 is an example of a microfluidic flow chemistry synthesis for the step of growing silver seeds into silver nanoparticles (step (b)) of the process for the production of discrete high definition silver nanoparticles;

FIG. 3A is a schematic of a microfluidic system using a generic microfluidics chip for silver seed production (step (a));

FIG. 3B is a detailed schematic of the generic microfluidics chip used in the system of FIG. 3A;

FIG. 4 is a schematic of an alternative microfluidic chip system for silver seed production (step (a));

FIG. 5 is a schematic of a microfluidic chip set up showing a sequence for reagent input in a process for producing nanoparticles;

FIG. 6 is a schematic of a microfluidics chip set up showing reagent output sequencing specifically designed for discrete high definition silver nanoparticle synthesis;

FIG. 7 is a schematic of a microfluidics chip showing the design requirements for stream input and mixing criteria;

FIG. 8 (A) is a graph showing the UV-visible spectrum for nanoparticles produced; (B) is a TEM micrograph of the nanoparticles produced; (C) is a bar chart showing the size distribution of the nanoparticles produced; and (D) are bar charts showing the distribution of shaped hexagons and triangles/truncated triangles in the nanoparticles produced. The discrete silver nanoparticles were produced from seeds synthesized using a generic microfluidic chip system with a flow rate ratio of 8:1 for solution 1 and 2 (step (a)) and conventional wet chemistry was used to grow the seeds into nanoparticles (step (b));

FIG. 9 (A) is a graph showing the UV-visible spectrum for nanoparticles produced; (B) is a TEM micrograph of the nanoparticles produced; (C) is a bar chart showing the size distribution of the nanoparticles produced; and (D) are bar charts showing the distribution of shaped hexagons and triangles/truncated triangles in the nanoparticles produced. The discrete silver nanoparticles were produced from seeds synthesized using a generic microfluidic chip system with a flow rate ratio of 8:1 for solution 1 and 2 (step (a)) and conventional wet chemistry was used to grow the seeds into nanoparticles (step (b)).

FIG. 10 is a line graph showing the UV-Visible spectrum of two sets of silver seeds produced with a flow rate ratio of 8:1 of solution 1 and 2 respectively (the two lines are superimposed);

FIG. 11 (A) is a line graph showing the main optical absorption spectrum peak width dependence (defined as full width at half maximum (FWHM)) and (B) is a line graph showing the dependence of the maximum wavelength for silver seeds produced using a microfluidic chip system with a variation in the flow rate of solution 1 (AgNO₃ and TSC) from 3 to 10 ml/min while keeping the flow rate of solution 2 constant at 1 ml/min;

FIG. 12 (A) is a graph showing the UV-visible spectrum for nanoparticles produced; and (B) is a TEM micrograph of the nanoparticles produced. The discrete silver nanoparticles were produced from seeds synthesised by conventional wet chemistry (step (a)) and the seeds were grown into nanoparticles using a microfluidics processor (step (b));

FIG. 13 (A) is a graph showing the UV-Visible spectra for silver seeds synthesised in presence of PSSS using a microfluidics processor method (processed seeds); discrete silver nanoparticles prepared by a conventional wet chemistry growth in presence of PSSS (step (b)) of microfluidics synthesised silver seeds (step (a)) (beaker experiment); and discrete silver nanoparticles produced from microfluidics synthesised silver seeds (step (a)) and microfluidics grown nanoparticles (step (b)) (microfluidics reaction technology). (B) is a graph showing the UV-Visible spectra of a silver seed solution synthesised using a conventional wet chemistry method (seeds); discrete silver nanoparticles prepared using a conventional wet chemistry method for both steps (a) and (b) (beaker experiment); and discrete silver nanoparticles prepared using a microfluidics process for both steps (a) and (b) (microfluidic reaction technology);

FIG. 14 is a graph showing the UV-Visible spectra for a range batches of discrete silver nanoparticles prepared under the same conditions by conventional wet chemistry method for both steps 1 and 2 (each line represents a different batch);

FIG. 15 (A) to (C) are TEM images for a range of discrete silver nanoparticles prepared under the same conditions by conventional wet chemistry method for both steps 1 and 2;

FIG. 16 is a graph showing the UV-Visible spectra for a range of batches of silver seeds (step (a)) synthesised by a conventional wet chemistry method (each line represents a different batch).

FIG. 17 A) is a TEM image of flat-lying silver nanoprisms from a typical sample. B) is a TEM image of silver nanoprisms from another sample, made by the same procedure, that are stacked together and are oriented such that they are standing vertically on their edges. C) is a UV-Vis spectrum of sample of nanoprisms shown in (A) showing the main SPR (in-plane dipole) at ˜825 nm;

FIG. 18 A) is a TEM image from a sample of flat lying silver nanoprisms grown from seeds produced with PSSS present. B) and C) are TEM images of silver nanoparticles grown from seeds produced without PSSS present, the sample comprises nanoprisms of a wide range of sizes and “spherical” particles of various sizes with 5-fold symmetry present. A “nanotape” is also visible in (C). D) is a UV-Vis spectra of nanoprisms in (A) grown from seeds produced in presence of PSSS (labeled A) and nanoparticles in (B) and (C) grown from seeds produced in absence of PSSS (labeled B);

FIG. 19 A) is a UV-Vis spectra of 4 samples (1 to 4) of nanoprism. B) is a plot of size data for silver nanoprisms from each of the 4 samples of (A). C) is a plot of data (squares) for the position of the main SPR (in-plane dipole) against edge-length (L) divided by thickness (T) for the four samples of (A). The dashed line is a linear fit to the data. The edge-length and thickness were obtained by measuring the dimensions of vertically oriented nanoprisms. The average thickness (T) of all samples is approximately 5.5 nm. The error bars represent the uncertainty in L/T that arises for the standard deviation of measurements of the edge-length and thickness of the nanoprisms;

FIG. 20 A) is a photograph of a series of samples (1 to 10) illustrating the range of colours obtained. The purple color in sample 10 is largely the result of extinction by the in-plane quadrupole. B) is a normalized spectra of a series of as prepared samples obtained using different volumes of seed solution: 1) 650 μl, 2) 500 μl, 3) 400 μl, 4) 260 μl, 5) 200 μl, 6) 120 μl, 7) 90 μl, 8) 60 μA 9) 40 μl, 10) 20 μl. C) is a spectra from (B) plotted against energy;

FIG. 21A) is a graph showing the FWHM of the SPRs plotted against plasmon resonance energy for each of the nanoprism samples (1 to 10). B) is a graph showing the FWHM of the SPRs plotted against nanoprism volume for each of the nanoprism samples (1 to 10). A standard deviation of 19% (based on data in Table 3) for edge-length and 1 nm uncertainty for thickness was used to generate the error bars for nanoprism volume;

FIG. 22 is a TEM image of a flat-lying nanoprism. A close-up in the bottom left clearly shows the 2.5 Å spacing between lattice fringes. The inset at bottom right is a Fourier transform of the whole image;

FIG. 23 is a schematic illustrating how intrinsic stacking faults along <111>, i.e. faults in the successive stacking of the ABC layers ({111} planes) of an fcc crystal, give rise to a hcp region. The black dots represent atoms in the {110} plane while the grey dots represent atoms immediately below;

FIG. 24 A) is a TEM image of a stack of vertically oriented silver nanoprisms. B) is a high resolution image of the nanoprism on the right hand side of (A) showing defect structure. This nanoprism is oriented such that the {110} plane is in the plane of the image, i.e. the electron beam is along <110>. C) shows the analysis of internal structure of nanoprism in (B). A series of intrinsic stacking faults has resulted in a hexagonally close packed pattern emerging and gives rise to an arrangement of atoms that is aligned perpendicular to the surface with a spacing of 2.50 Å. The correct spacing of 2.35 Å has been obtained for {111} planes and also for the alternate ABAB . . . layers of the hcp region;

FIG. 25 is a graph showing X-ray diffraction data for silver nanoprisms, the peaks corresponding to fcc silver are labeled with * and the relevant miller indices. Two additional peaks (labeled with x) correspond to predicted positions from theoretical diffractograms, reported in the Supporting Information Section of reference 57 for a defect-induced hcp arrangement of silver atoms in silver nanoparticles;

FIG. 26 A) is a schematic illustrating a <110> oriented segment of a fcc crystal. The edges of a crystal cut in this manner have alternating pairs of {100} and {111} faces. B) is a schematic of a nanoplate constructed from a single fcc crystal (no twin planes or defects). A singe crystal would not normally take up this structure but the schematic illustrates that a nanoplate cut from a fcc crystal could have edges consisting of alternating pairs of {100} and {111} faces. C) is a schematic of a nanoplate with a defect-induced hcp layer sandwiched between two fcc layers of unequal thicknesses. The hcp layer determines the lateral growth. Within the two-dimensional growth plane, certain directions are preferred due to the asymmetric distribution of crystal faces. The block arrows indicated the proposed directions of preferred growth that lead to the familiar triangular shape of nanoprisms;

FIG. 27 shows TEM images of flat-lying and stacked silver nanoprisms for samples 1 to 4. There is a clear trend of increasing edge-length of nanoprisms with the spectral position of the main SPR as shown in FIG. 19A. Scale bars are 20 nm;

FIG. 28 is a spectra of sample 10 from FIG. 20B (line A) and of sample 4 from FIG. 19A (line B). The in-plane quadrupole SPRs are clearly visible with a shoulder at ˜465 nm that is tentatively assigned as an in-plane octupole SPR. The out-of-plane dipole and quadrupole SPRs are visible at ˜400 nm and ˜330 nm respectively;

FIG. 29 is a line graph showing the UV-Visible spectrum of silver seeds produced by microfluidics with a flow rate ratio of 1:1 of solution 1 (AgNO₃) and solution 2 (NaBH₄, PSSS and TSC) respectively; and

FIG. 30 is a graph showing the UV-visible spectrum of size and colour tuned (spectral peak ranging from 656 nm to 500 nm) triangular silver nanoplates produced by varying the volume of generic microfluidic produced seeds from 100 μl to 600 μl in steps of 100 μl or 50 μl as listed in table 4.

DETAILED DESCRIPTION

We describe an improved process for producing nanoparticles, which is suitable for the production of size-controlled nanoparticles having controlled dominant shapes, such as silver triangular nanoplates. The process for producing discrete high definition silver nanoparticles comprises two steps: step (a) synthesizing silver seeds and step (b) growing silver seeds into silver nanoparticles.

The invention provides a microfluidic process for producing discrete high definition silver nanoparticles. Microfluidic technologies can be applied to at least the growth step (step (b)) or to both the silver seed production and growth steps (steps (a) and (b)).

By using microfluidic methods, we can produce discrete high definition silver nanoparticles in a predetermined and controlled manner. The nanoparticles produced using the processes described herein are highly shaped, e.g. contain a high percentage of triangles and hexagons compared to spheres, and/or have a narrow size distribution in a desired size range such as about 25 nm or about 30 nm or about 40 nm or larger or smaller and/or have a UV-visible optical absorption spectrum with a main peak at wavelengths longer that about 400 nm. The full width at half maximum (FHWM) of the main peak may be less than about 100 nm.

By employing a combination of both microfluidics chip and microfluidics processor methods for steps (a) and (b) we have devised a process that enables the scaled-up production of discrete high definition silver nanoparticles. The silver nanoparticles produced by the microfluidic process described herein have a high batch to batch reproducibility and improved physical properties including a narrower size distribution, an increased presence of shaped nanoparticles and a higher uniformity between the silver nanoparticles. The microfluidic methods also allow the size, shape, spectral profile and surface chemistries of the discrete high definition silver nanoparticles to be controlled. The microfluidic synthesis processes described herein can be adapted for the scaled up production of a range of high quality nanoparticles both metallic and non metallic.

It will be appreciated that scaled up production requires larger volumes of reagents and for the synthesis process to be successful, the reagents have to be thoroughly mixed. We have surprisingly found that in order to produce nanoparticles having a controlled shape and size the reagents should be mixed in small volumes such as between about 10 picoliters (pl) to about 100 μl, microfluidic methods are ideal for the thorough and rapid mixing of reagents in such small volumes.

In some embodiments, mixing of the reagents may be performed in small volumes in a microfluidic reactor at high or differential flow rates. For example at flow rates between about 1 ml/min to about 10 ml/min for low pressure systems and flow rates of at least 10 ml/min up to litres/min for high pressure systems. The reagents used in step (a) and/or step (b) of the process may have differential flow rates. The flow rate of individual reagents can be variably controlled within a microfluidic reaction system resulting in the reagent solutions being rapidly and thoroughly mixed.

We have also found that the ratio of the reagents, and/or the ratio at which the reagents are mixed can impact the physical properties of the nanoparticles formed. For example, an excess of about eight times the reducing agent solution to the silver salt solution has been found to be optimum for the reaction chemistry for producing silver seeds in step (a) of the process.

The performance of certain aspects of the reaction chemistry, for example in step (b) of the process may require a microfluidic reactor which is capable of delivering reagent solutions under high pressures for example between about 35 MPa to about 275 MPa (about 5000 psi to about 40000 psi), such as about 140 MPa (about 20000 psi). Mixing reagents under pressure in step (a) and/or step (b) of the process may assist with the rapid and thorough mixing of the reagents.

The use of high pressure flow and/or variable differential flow rates of reagents may allow for a uniform reaction to take place. The pressure and flow rate of reagents and the dimensions of the microfluidic reactor may be such that a turbulent flow of reagents is generated at the point at which the reaction takes place. Turbulent flow of reagents may thereby promote thorough mixing of the reagents and maintain consistent control of the reaction chemistry in a continuous microfluidic flow process. By maintaining consistent control of the reaction chemistry, we have been able to produce silver nanoparticles having physical characteristics within a well defined process envelope.

The microfluidic reactor, when designed and operated as described herein will maintain a continuous flow and through mixing of reagents under controlled conditions thereby allowing a true scaling up of the reaction chemistry without compromising the quality of the nanoparticles produced.

Advantageously, the thorough and rapid mixing of reagents according to the process described herein allows for certain desired characteristics of the silver nanoparticles to be controlled and reproducible produced. Such controlled reproducibility is not possible in a conventional wet batch chemistry reaction in which reagents are mixed in higher volumes compared to the microfluidic process resulting in variations in nanoparticle characteristics both within a batch, and between batches.

Steps (a) and (b) can be combined in some embodiments of the invention to produce a single step microfluidic production method for nanoparticles.

Depending on the properties of the nanoparticles desired, the order of addition of the reagents, the type of reagents used, and/or the concentration of the reagents can all be varied and in some embodiments additional reagents can be introduced into the reaction. By varying these reagent parameters the physical properties and attributes of the nanoparticles, such as their size, shape, thickness and their optical spectrum can controllably tuned.

The microfluidic methods described herein enable the reproducible production of high definition silver nanoparticles with predetermined, size, shape, narrow distribution of size and shape.

We have demonstrated that a microfluidics processor method can be used to produce discrete high definition silver nanoparticles in large volume batches. By tailoring pressure and shear rate parameters, silver nanoparticles can be produced on an industrial scale while retaining control of the reaction chemistry conditions necessary to produce controlled size and shape range silver nanoparticles. We used a high pressure (for example in the range of about 35 MPa to about 275 MPa, such as about 140 MPa), high shear rate (for example between about 1×10⁶ s⁻¹ to about 50×10⁶ s⁻¹, typically about 10⁷ s⁻¹) microfluidics processor method to produce silver nanoparticles in 500 ml batches. The process is capable of producing several liters per hour at flow rates typically in the range of about 10 ml/minute to about 500 ml/minute, while a wet chemistry method is limited to 100 ml batch production.

The invention is further illustrated with reference to the following non-limiting examples.

EXAMPLES Example 1 Microfluidic Production of Silver Seeds

We have found that by using microfluidic technologies for the production of silver seeds control over the synthesis of the silver seeds is an important factor in producing discrete high definition silver nanoparticles with predetermined, size, shape and a narrow distribution of size and shape

FIG. 1 is a schematic illustrating the set up for microfluidic synthesis of silver seeds.

The constituent chemicals and products may vary from those detailed in FIG. 1 wherein product 1 is a silver nitrate (AgNO₃) and Trisodium Citrate (TSC) solution and product 2 is a sodium borohydride (NaBH₄) solution. Briefly, referring to FIG. 1, a silver source (in this case silver nitrate) is mixed with trisodium citrate at about 0° C. Following mixing sodium borohydride (NaBH₄) is added to the AgNO₃-TSC solution and the mixture is incubated at about 0° C.

In an alternative process, product 1 is a sodium borohydride (NaBH₄) and Trisodium Citrate (TSC) solution and product 2 is a silver nitrate (AgNO₃) solution. The microfluidics set-up for the production of the silver seeds is as shown in FIG. 3A, this consists of a microfluidic reactor chip system (micromixer glass or polymer chips) to which the component solutions, such as those described in FIG. 1 are added at a controlled rate using pumps. The microfluidics chip may be of a generic type, i.e. an “off the shelf” chip. Details of a suitable generic chip are given in FIG. 3B and in Table 1 below.

Optionally, a polymer such as poly(sodiumstyrene sulfonate) (PSSS) may be added to step (a). For example, PSSS could be included in one or more of the silver nitrate solution, trisodium citrate solution, and sodium borohydride solution at a concentration of about 10⁻⁴ M.

TABLE 1 Suitable parameters of microfluidic chip Chip internal volume 250 μl Pressure rating 30 Bar (450 psi) Pressure drop across chip for 0.2 Bar water flowing at 100 μl/min Material B270 Number of glass layers  2 Channel fabrication Double isotropic etch and thermal bond Channel cross-section Rectangular, curved along shorter dimension. In some preferred embodiments approximately 3:1 width to height ratio

Hole fabrication Mechanical drill Channel shape Circular Channel depth/μm 250 Mixing channel width/μm 300 Mixing channel length/mm 532 Mixing channel pitch/μm 500 Reaction channel width/μm 400 Reaction channel length/mm 250 Reaction channel pitch/μm 600

Example 2 Protocol for the Production of Silver Seeds Using a Microfluidic Chip System

Referring to FIGS. 3A and 4, dissolve 37.8 mg of sodium borohydride in 100 ml of water (Solution 1 of FIG. 3A).

Dissolve 5 mg of silver nitrate and 7.4 mg of trisodium citrate in 100 ml of iced cooled water in an ice bath (solution 2 of FIG. 3A).

Connect solution 1 and solution 2 to pump 1 and pump 2 respectively (see setup of FIGS. 3A and 4).

Set pump 1 and pump 2 flow rates for example at 1 ml/min and 8 ml/min respectively;

-   -   Run pump 1 for 30 s and collect by-product;

Run pump 2, while pump 1 is still running and collect by-product for 30 s.

Collect 5 ml of final seed product, while pump 1 and pump 2 are still running.

Stop both pump 1 and pump 2.

A more generic setup for reagent input sequencing for general nanoparticle production is shown in FIG. 5. This setup can be applied to the production method for of a wide range of nanoparticles including high definition silver nanoparticles. For general nanoparticle production the setup, conditions and reagents would need to be adjusted for each particular type of nanoparticle to be produced.

Example 3 Experimental Results for Application of Microfluidics Methods to Silver Seed Production

The results from experiments using a generic microfluidic chip system for the production of silver seeds (step (a)) are given below. In these cases the second step, (the growth of these seeds to produce discrete high definition silver nanoparticles) was carried out using a conventional batch chemistry method.

In this example, silver seeds were synthesised using a generic microfluidic chip system according to the following method:

37.8 mg of sodium borohydride (0.01M) was dissolved in 100 ml of water (solution 1 of FIG. 3A). 5 mg of silver nitrate (2.94×10⁻⁴M) and 7.4 mg of trisodium citrate (2.5×10⁻⁴M) were dissolved in 100 ml of iced cooled water in an ice bath (solution 2 of FIG. 3A). Solution 1 and solution 2 were connected to pump 1 and pump 2 respectively (as shown in the setup of FIGS. 3A and 4). The flow rate of pump 1 was set at 1 ml/min under a pressure of about 2 MPa (20 bar). The flow rate of pump 2 was set at 8 ml/min under a pressure of about 2.5 MPa (25 bar).

Pump 1 was run for 30 s and the by-product collected. With pump 1 still running, pump 2 was run for 30 s and the by-product collected. Prior to stopping the pumps, 5 ml of final seed product was then collected while pumps 1 and 2 were still running.

In this example silver seeds were grown into nanoparticles (step (b)) using the conventional wet chemistry method below.

45 ml of 1 wt % polyvinyl alcohol (PVA) and 1.25 ml of 0.01M silver nitrate (AgNO₃) were placed in a 400 ml beaker equipped with a 5 cm magnetic stirrer.

The beaker was placed on a hot plate set at 40° C. and the solution was stirred for 45 minutes in the dark. 0.5 ml silver seed solution from step (a) above was diluted with 5 ml PVA and added to the PVA-AgNO₃ solution. Approximately 30 s after the silver seed solution was added to the PVA-AgNO₃ solution 250 μl of 0.1M ascorbic acid was added to the mixture in one rapid shot.

FIG. 8 shows the UV-visible spectrum (A) and a TEM image (B) of discrete high definition silver nanoparticles produced using seeds produced by the generic microfluidic chip with a flow rate ratio of 8:1 for solution 1 (AgNO₃ and TSC) and 2 (NaBH₄). The average nanoparticle size is about 21.6±7.5 nm, with about 75.4% of the nanoparticles being shaped, for example, triangular, truncated triangular and hexagonal. The full width at half maximum (FWHM) of the main UV-visible spectral peak is about 105 nm. The peak maximum wavelength is in the region of about 520 nm.

FIG. 9 shows the UV-visible spectrum (A) and a TEM image (B) of a different batch of discrete high definition silver nanoparticles produced using seeds produced by the generic microfluidic chip with a flow rate ratio of 8:1 for solution 1 (AgNO₃ and TSC) and 2 (NaBH₄). The average nanoparticle size is about 24.0±8.9 nm, with about 44.9% of the nanoparticles being shaped, for example, triangular, truncated triangular and hexagonal. The FWHM of the main UV-visible spectral peak is about 120 nm. The peak maximum wavelength is in the region of about 519 nm.

The nanoparticles of FIGS. 8 and 9 demonstrate the ready reproducibility of using a microfluidic process for synthesising silver seeds (step (a)). The nanoparticles grown from the solver seeds are discrete high definition silver nanoparticles with similar size, narrow size distribution and a high percentage of shaped nanoparticles, using a microfluidic method to produce the silver seed nanoparticles.

FIG. 10 Shows the UV-visible spectra of two different sets of microfluidic seeds produced using a generic microfluidic chip with a flow rate ratio of solution 1 and 2 of 8:1. The UV-visible spectra for each set of microfluidic synthesisied seeds are superimposed demonstrating the reproducibility of the microfluidic method for synthesising silver seeds with controlled physical properties.

FIG. 11 shows the dependence of seed FWHM and maximum wavelength with variation of flow rate of solution 1 (AgNO₃ and TSC) from about 3 ml/min to about 10 ml/min while keeping flow rate of solution 2 (NaBH₄) constant at 1 ml/min. In both the case of FWHM and maximum wavelength, an increase is observed as the flow rate of solution 1 (AgNO₃ and TSC) is increased from about 3 ml/min to about 7 ml/min, follow by a sharp dip at about 8/ml per min and a subsequent recovery to the increasing trend from about 9 ml/min to about 10 ml/min.

Example 4 Microfluidic Growth of Silver Nanoparticles from Silver Seeds

We used a microfluidics processor to carry out step (b), the growth of silver seeds to produce high quality discrete high definition silver nanoparticles. In this Example, the silver seeds were synthesised using a conventional wet chemistry method.

We used the Microfluidics International Corporation microfluidics processor technology described in Example 4 above to create a 500 ml batch of discrete high definition silver nanoparticle solution. Referring to FIGS. 12A and B, the average nanoparticle size was about 37±18 nm, with about 31% of the nanoparticles being shaped, for example, triangular, truncated triangular and hexagonal. The FWHM of the main UV-visible spectral peak was about 98 nm.

Blocking and clogging difficulties were encountered in some experiments when a microfluidic chip systems was used for carrying out step (b), the growth of discrete high definition silver nanoparticles from silver seeds. It was found that blocking and clogging of the microfluidic system could be overcome if the reagents were under pressure for this step. A limited number of suitable commercial microfluidics processors are available, and have heretofore been used for processes other than chemical reactions. In this Example we used a microfluidics system supplied by a company now known as Microfluidics International Corporation located at 30 Ossipee Road, P.O. Box 9101 Newton, Mass. 02464-9101, U.S.A. This microfluidics processor operates at very high pressures of the order of about 140 MPa (about 20,000 psi) and provides high shear rates in the range of about 1×10⁶ s⁻¹ to about 50×10⁶ s⁻¹, thereby maximizing the energy-per unit fluid volume.

The microfluidics processor used allowed the reagent streams to be pressurized so that the reagent streams traveled at high velocities to meet in a reaction chamber where turbulent mixing took place. The microfluidics processor also allowed for continuous flow of the reaction product (silver nanoparticles). Details of typical processor operating parameters are given in Table 2 below.

TABLE 2 Suitable parameters of microfluidics processor Pressure range 35 MPa to 275 MPa (5,000 psi to 40,000 psi) Flow rate range 10 ml/min to liters/min Typical fluid velocity 1.2-20 m/s up to 500 m/s Typical resident time 0.5-1 ms down to 2 μs Typical shear rate 1-50 × 10⁶ s⁻¹

FIG. 2 shows a microfluidic system set up for growing nanoparticles from silver seeds. Whilst it will be appreciated that the constituent chemicals and products may vary, in this Example product 3 is a silver nitrate (AgNO₃) polyvinyl alcohol (PVA) solution, product 4 is a silver nitrate (AgNO₃) polyvinyl alcohol (PVA) and silver seed solution, and product 5 is a solution of the discrete high definition silver nanoparticles.

A TEM image of the silver seeds produce (product 5) is shown in FIG. 12B.

Example 5 Application of Microfluidics Methods to Both Silver Seed Production and Silver Seed Growth for the Production of Discrete Silver Nanoparticles

The Microfluidics International Corporation microfluidics processor technology described in Example 4 above was also applied to the production of silver seeds (step (a)) and in a further stage these microfluidic processor produced seeds were grown to produce discrete silver nanoparticles (step (b)) also using a microfluidics processor.

In this example the following method was used:

Step (a)—Synthesising Silver Seeds

A solution comprising 2.94×10⁻⁴M AgNO₃ and 2.5×10⁻⁴M TSC and 10⁻⁴M PSSS in water was made and poured into the reservoir of a microfluidics processor. A 0.01M solution of NaBH₄ was introduced into the microfluidics processor. The NaBH₄ and AgNO₃-TSC solutions were mixed at flow rates of 15 ml/min and 485 ml/min respectively with a continuously flowing stream of the AgNO₃-TSC solution and the material was processed for one pass at 140 MPa (20,000 psi).

Step (b)—Growing Silver Nanoparticles

An aqueous solution of 1 wt % PVA, 0.01M AgNO₃ and 10⁻⁴ M PSSS in water was made in a beaker equipped with a magnetic stir bar (the total volume was 500 ml). The beaker was placed on a hot plate set at 40° C. and stirred for 45 minutes in the dark. 5 ml if silver seed solution from step (a) above was diluted in 50 ml PVA and added to the beaker.

Approximately 30 s after the silver seed solution was added to the beaker, the PVA-AgNO₃-PSSS-silver seed solution was placed in the reservoir of a microfluidics processor. A 0.01M solution of ascorbic acid solution was introduced to a 475 ml/min continuously flowing stream of PVA-AgNO₃-PSSS-silver seed solution at a rate of 25 ml/min. The material was processed for 1 pass at 35 MPa (20,000 psi).

The UV-visible spectrum of silver seeds produced using a microfluidics processor and the discrete high definition silver nanoparticles produced by the subsequent growth of these microfluidics processor produced silver seeds also using a microfluidics processor are shown in FIG. 13A. Also shown are discrete silver nanoparticles produced by the conventional wet chemistry growth of the microfluidic processor synthesized silver seeds. It is clear from the spectra shown in FIG. 13A that the microfluidic process of growing the microfluidic synthesized silver seeds (i.e. using a microfluidics process for both steps (a) and (b)) results in the production of discrete silver nanoparticles with a much higher presence of shaped silver nanoparticles compared to a conventional wet chemistry operation of the growth step as is signified by the much more distinct peak in the region of 345 nm in the case of the microfluidics processor produced discrete silver nanoparticles.

Referring to FIG. 13B, a silver seed solution was synthesised using a conventional wet chemistry method and discrete silver nanoparticles were prepared by either using a conventional wet chemistry method for growing the wet chemistry synthesised silver seeds or a microfluidic processor for growing microfluidic synthesized silver seeds (i.e. using a microfluidics process for both steps (a) and (b)). It is clear from the spectra shown in FIG. 13B that using a microfluidics process for both steps (a) and (b)) results in the production of discrete silver nanoparticles with a much higher presence of shaped silver nanoparticles compared to a conventional wet chemistry method.

Example 6 (Comparative Example) Wet Cheinisny Batch Nanoparticle Production Both Step (a) and Step (b)

A wet chemistry method was used to synthesize silver seeds (step (a)) and growing the silver seeds to form discrete silver nanoparticles (step (b)), as described in WO04/086044. Briefly, silver seeds were formed from vigorously stirring an aqueous mixture of silver nitrate, trisodium citrate and sodium borohydride. The typical ratio of silver nitrate: trisodium citrate was about 1:1 and the typical ratio of silver nitrate: sodium borohydride was about 1:8.

The wet chemistry method has a restricted production volume in the order of about 50 ml, with a maximum of up to about 100 ml of discrete silver nanoparticles being produced in any one batch. Batch to batch reproducibility difficulties are experienced, as indicated by the diverse range of UV-Visible spectra of discrete silver nanoparticles using wet chemistry prepared under precisely the same conditions as shown in FIG. 14. These batches of discrete silver nanoparticles have spectra, whose maximum peak wave lengths range over 70 nm, have FWHM in excess of 150 nm and have spectra which vary between single peaked to twin peaked where both spherical and shaped associated peaks are of similar intensity to the case where the shaped associated peak is dominant.

FIG. 15 (A) to (C) show representative TEM images of discrete silver nanoparticles prepared using a wet chemistry method for both steps (a) and (b). FIGS. 15 (A) and (B) illustrate the wide size distribution within and between batches and FIG. 15 (C) illustrates the low presence of shaped nanoparticles.

FIG. 16 shows UV-visible spectra for three different batches of silver seeds produced under the same conditions using conventional wet chemistry. The spectra profiles are very similar with a peak maximum wavelength in the region of 399 nm and a FWHM of the order of 65 nm. These silver seeds are typical of those used in the wet chemistry step (b) resulting in discrete silver nanoparticles which have the range of variation and poor reproducibility shown in FIGS. 14 and 15.

This is in direct contrast to the results for the microfluidic methods for discrete silver nanoparticle production where discrete silver nanoparticles with narrow size distribution, high percentage of shaped nanoparticles and very similar UV-visible spectral profiles with peak maximum wavelengths within 1 nm can be readily prepared, as indicated by the discrete silver nanoparticles shown in FIGS. 8 and 9.

We have found major differences in the performance of the wet chemistry and microfluidic produced silver seeds in terms of, reproducibility, control over size, shape, size and shape distribution, percentage of shaped and unshaped nanoparticles present in discrete silver nanoparticle batches produced subsequently from the seeds, either by employing conventional wet chemistry or microfluidics methods to carry out step (b), the growth of the silver seeds to from discrete silver nanoparticles. We have found that the control and precision afforded by microfluidics methods for the production of silver seeds is important in achieving discrete silver nanoparticles with the required characteristics, controlled size, narrow size distribution, high presence of shaped nanoparticles and good batch to batch reproducibility. Thus, microfluidics methods, such as microfluidics processors, can be readily applied to produce litres per hour of the discrete silver nanoparticles with out sacrificing quality.

We describe a rapid and readily reproducible seed-based method for the production of high quality silver nanoprisms in high yield (at least 95%). The edge-length and the position of the main plasmon resonance of the nanoprisms can be readily controlled through adjustment of reaction conditions. From UV-Vis spectra of solutions of the nanoprisms, the inhomogeneously broadened line width of the in-plane dipole plasmon resonance is measured and trends in the extent of plasmon damping as a function of plasmon resonance energy and nanoprism size have been elucidated. In-depth analysis of the lamellar defect structure of silver nanoprisms confirms that the defects can lead to a transformation of the crystal structure in the vicinity of the defects. These defects can combine give rise to lamellar regions, thicker than 1 nm, that extend across the crystal, where the silver atoms are arranged in a continuous hcp structure. This hcp structure has a periodicity of 2.50 Å, thus explaining the 2.50 Å lattice fringes that are commonly observed in <111> oriented flat-lying nanoprisms.

Nanoparticles of noble metals such as silver are of considerable interest in nanotechnology. This stems largely from the collective oscillation of the conduction electrons in resonance with certain frequencies of incident light, leading to an extinction known as a surface plasmon resonance (SPR).^([1,2,3,4,5]) The spectral position of the resonance is highly dependent on nanoparticle size and shape and also depends on the refractive index of the metal and the surrounding medium.

One of the key, and most interesting, properties of highly-shaped metal nanoparticles is the fact that at the SPR of a metal nanoparticle, the electric field intensity near the surface of the nanoparticle is enhanced strongly relative to the applied field.^([1,6]) Two potential applications of this field enhancement are Surface Enhanced Fluorescence (SEF)^([7,8,9,10,11,12]) and Surface Enhanced Raman Spectroscopy (SERS).^([13,14]) The degree of enhancement is dependent on a number of factors. One of these is shape. It has been shown by discrete dipole approximation (DDA) calculationst^([6]) that nanorods and nanoprisms show a much higher degree of enhancement of the local field than spheres. Recently, electron energy-loss spectroscopy (EELS)^([15,16]) has permitted high-resolution probing of the SPR on metal nanorods and nanoprisms and has generated results consistent with the optical spectra and calculations.

Another factor that affects the field enhancement is damping of the surface plasmon, which is characterized by the dephasing time, T₂. The field enhancement factor, if is directly proportional to the dephasing time T₂, of the SPR (|f|αT₂), where T₂=2 h/Γ_(hom) and Γ_(hom) is the homogeneous line width.^([17,18,19]) Damping of the plasmon occurs through either non-radiative decay (absorption), or transformation of the plasmon into photons (scattering), known as radiation damping, i.e. Γ_(hom)=Γ_(rad)+Γ_(non-rad).^([17,18,20,21]) Accordingly, the suitability of certain nanoparticle morphologies for applications that rely on field enhancement can be estimated from measurements of the homogeneous line width of individual nanoparticles. For example, a series of experiments comparing nanospheres and nanorods has shown that nanorods typically display dramatically reduced plasmon damping compared to spheres, i.e. narrower line widths,^([18]) and therefore produce a stronger field enhancement. At low plasmon resonance energies this difference is a result of the nanorods exhibiting much lower radiation damping. This is because the nanorods have a much lower volume than the corresponding nanospheres with the same plasmon resonance energy, and the radiative dephasing rate (radiation damping) is proportional to nanoparticle volume,^([18,19,20,22]) i.e. Γ_(rad)αV. Since different nanoparticle shapes result in different nanoparticle volumes for a given plasmon resonance energy, it is clear that the degree of plasmon damping is highly influenced by nanoparticle shape and this is another route for nanoparticle shape to influence the degree of enhancement of the local field. The non-radiative contribution to plasmon damping increases with increasing plasmon resonance energy due to the frequency-dependent dielectric properties of silver.^([19,20,23])

Similarly, more recent experiments have shown that line widths for the SPRs of Au—Ag nanoboxes are much broader than those of gold nanorods with comparable plasmon resonance energies.^([24]) In addition, Ginger and co-workers have studied the line widths of scattering spectra of individual silver nanoprisms and found that the line widths increase both as the particle volume increases and as the plasmon resonance energy increases.^([20])

Potential applications such as SEF and SERS are the driving force for the development of synthetic approaches that involve a high-degree of control over the final nanoparticle morphology. Silver nanoprisms have received considerable attention as the in-plane dipole SPR can be tuned across the entire visible spectrum from ˜400 nm to the near infra-red (NIR). The syntheses that exist for the production of silver nanoprisms can be generally placed into either of two categories: photochemical (plasmon-driven synthesis)^([25,26,27,28,29]) and thermal.^([13,30,31,32,33,34,35,36,37,38,39]) Photochemical syntheses have produced the highest quality samples to date but this approach typically involves days for the preparation of a sample. Thermal approaches are much quicker but often produce samples with a range of shapes and sizes.

There has been extensive research investigating the different factors that influence particle size and shape. Until recently, some explanations for the existence of anisotropic growth in an isotropic medium were based upon the assembly of surfactant molecules into a template whose shape then defines the growth of the crystal,^([40,41]) particularly for nanorods and nanowires. It has been more commonly thought that there is preferential adsorption of organic molecules, such as polymers and surfactants, to less stable crystal faces such as {100} and {110}. In this model, there is a much faster rate of addition of metal atoms at to the more exposed faces, resulting in preferred growth directions. For example the preferred binding of polyvinylpyrrolidone (PVP) to the {100} side faces of decahedral silver nanoparticles leading to silver nanowires with 5-fold symmetry due to growth on the {111} end faces^([42]) and to the {100} faces of single crystal silver nanoparticles leading to silver nanocubes as a result of the faster growth on the {111} faces;^([43,44]) the preferential adsorption of cetyltrimethylammonium bromide (CTAB) surfactant on the {100} side faces of decahedral gold nanoparticles leading to gold nanowires with 5-fold symmetry due to growth on the {111} end faces;^([45,46,47]) the preferential adsorption of CTAB-Ag⁺ on the {100} and {110} side faces of single crystal gold nanoparticles leading to single crystal nanorods due to preferred growth on the mostly {111} end faces;^([48,49,50]) the preferential adsorption of cetyltrimethylammonium tosylate (CTAT) on the {100} faces of decahedral silver nanoparticles leading to silver nanorods with 5-fold symmetry.^([51]) In addition, recent computational work has been successful in predicting anisotropic growth based on the face-selective binding of surfactants.^([52])

Nevertheless, even for these examples, it is clear that the anisotropic growth that results from the preferential binding of organic species to certain crystal faces relies on the underlying twinning or defect structure of the seed particles since this is what determines the type and orientation of the crystal faces that are exposed to the growth medium. This is all the more apparent when we consider that in most syntheses a range of particle shapes are observed and yet the same shaped particle can be the major product of very different syntheses. Furthermore, anisotropic structures such as nanoprisms present a particular challenge to the face-selective binding model in that gold and silver nanoprisms typically have large flat {111} faces, with two-dimensional growth from the edges. Many syntheses for nanoprisms take place in the presence of such stabilizers as PVP or surfactants, yet unlike the nanowire, nanorod and nanocube examples listed above, growth is restricted in the <111> direction. This would suggest that it is quite possible that the organic stabilizers that are often present in the syntheses of nanoprisms provide a general stabilization of the growing nanoprisms and may play little or no shape-directing role.

Indeed, the internal defect structure has been implicated as a direct factor influencing crystal growth. Specifically, defects such as twinning that arise during the early stages of particle formation give rise to preferred growth directions where the defects are exposed to the growth medium. In the case of nanoprisms, parallel stacking faults in the <111> direction have been observed with these making contact with the growth medium at the edges, precisely where growth occurs.^([53]) The silver halide growth model has also been resurrected as a way of explaining particle growth in many synthesis methods.^([54,55,56,57]) In this model, twin planes form reentrant grooves, which are favorable sites for the attachment of adatoms. A single twin plane is expected to direct growth in two dimensions but limit the final size of the nanoprism, while the presence of two parallel twin planes would allow the fast growing edges to regenerate one another, allowing shapes such as hexagonal nanoplates to form. Very recently, Rocha and Zanchet have studied the defects in silver nanoprisms in some detail and have shown that the internal structure can be very complex with many twins and stacking faults.^([58]) These defects are parallel to each other and the flat {111} face of the nanoprism, subdividing it into lamellae which are stacked in a <111> direction, and are also present in the silver seeds. In that paper, it was demonstrated how the planar defects in the <111> direction could give rise to local hexagonally close-packed (hcp) regions. These would in turn explain the 2.50 Å lattice fringes that are observed in <111> orientated nanoprisms, which have hitherto been attributed to formally forbidden ⅓{1422} reflections.^([52,53,54,59])

We describe a thermal synthetic procedure that selectively produces (>95%) silver nanoprisms in high yield. The as prepared samples are sufficiently monodisperse that important features of the SPR of silver nanoprisms are visible. Trends in the evolution of the degree plasmon damping as a function of plasmon resonance energy and nanoprism size can be elucidated through analysis of the UV-Vis spectra. The TEM data reveals that the defects have a significant impact on the crystal structure of silver nanoprisms and have important implications for understanding of the role of defects in the anisotropic growth mechanism for silver nanoprisms.

Example 7 Synthesis of Silver Nanoprisms Seed Production:

In a typical experiment, silver seeds are produced by combining aqueous trisodium citrate (5 ml, 2.5 mM), aqueous poly(sodium styrenesulphonate) (PSSS; 0.25 ml, 500 mg L⁻¹; Aldrich 1,000 kDa) and aqueous NaBH₄ (0.3 ml, 10 mM, freshly prepared) followed by addition of aqueous AgNO₃ (5 ml, 0.5 mM) at a rate of 2 ml min⁻¹ while stirring continuously.

Nanoprisin Growth:

The nanoprisms are produced by combining 5 ml distilled water, aqueous ascorbic acid (75 μl, 10 mM) and various quantities of seed solution, followed by addition of aqueous AgNO₃ (3 ml, 0.5 mM) at a rate of 1 ml min⁻¹. After synthesis, aqueous trisodium citrate (0.5 ml, 25 mM) is added to stabilize the particles and the sample is diluted with distilled water as desired. Distilled water is used throughout for all solutions. The synthesis is complete after the 3 minutes required for addition of the AgNO₃ during which time the colour of the solution changes as the SPR red-shifts in response to nanoprism growth.

Structural Characterization:

Samples were prepared for XRD measurements by concentrating a nanoprism sample by centrifugation. A viscous nanoprism mixture was prepared by adding the few drops of concentrated nanoprism solution to a few drops of aqueous 5% w/v poly(vinyl alcohol) (PVA). This was added to the glass slide for XRD analysis (Philips X′Pert Pro) and allowed to dry.

All TEM images were taken using a JEOL JEM-2100 LaB₆ at 200 kV.

We describe a method for silver nanoprism synthesis that is a seed-based thermal synthetic procedure that selectively produces (>95%) silver nanoprisms in a rapid and reproducible manner; and under very mild conditions (room temperature and water as solvent). The method involves the silver seed-catalyzed reduction of Ag⁺ by ascorbic acid, and surprisingly results in a minimal concentration of spherical nanoparticles being produced. The spectral position of the SPR can be tuned by controlling the size of the nanoprisms, without any significant variation in thickness. This can be achieved through adjustment of the number of seeds in the growth mixture.

A typical example of the nanoprisms produced with this method is shown in FIG. 17.

A key ingredient for production of high quality samples is poly(sodium styrenesulphonate) (PSSS), which is used as a stabilizer in the seed production step. If PSSS is left out or only added to the seed solution after seed production, then there is a diversity of nanoparticle shapes and sizes, this is shown clearly in FIG. 18. This result is important as it shows that the PSSS is not simply playing a shape-directing role through preferential adsorption to certain crystal faces during the growth stage, but rather it must have a strong influence on the defect structure of the seeds and indeed a preference for seeds whose structure predisposes them for growth into nanoprisms. We believe that as PSSS is a charged polymer, it interacts relatively strongly with the silver surface thereby influencing the defect structure of the seeds.

Generally, the amount of citrate present in the synthesis of many of the samples is very low. For example, in a synthesis that uses 100 μl of seed solution there is 118 nmol of citrate in the solution during the growth step, while 1,500 nmol of Ag⁺ is added. This contrasts with previously reported results which indicate that a low citrate/Ag⁺ ratio (<1) resulted in triangular and hexagonal structures with a broad range of sizes (30 to 300 nm) while a high citrate/Ag⁺ ratio (>1) was required for nanoprisms to be the major product.^([33]) This was reasoned to be the result of citrate likely effecting the face-selective growth by adsorbing more strongly to the flat {111} face of the nanoprisms. Here, analysis of samples by UV-Vis indicates that increasing the amount of citrate used in the synthesis does not increase the anisotropy of the nanoprisms. The high-quality nanoprisms we have obtained with relatively low quantities of citrate present indicate that it is more likely that the defect structure of the seeds, rather than the presence of citrate in the growth step, is the basis for anisotropic growth into nanoprisms. Referring to FIGS. 18B and C, when the amount of citrate is relatively high in the seed production step and even when PSSS is absent highly shaped nanoparticles are still obtained. Citrate may therefore play an important role in anisotropic growth by influencing the defect structure of the seeds. However when citrate is used in the absence of PSSS nanoparticles having a variety of different shapes are produced. The advantage of using PSSS is that nanoparticles having a predominantly triangular shape are produced.

To characterize the nanoprisms produced by this method and explore the relationship between nanoparticle dimensions and the position of the main SPR, TEM analysis of statistically significant numbers of nanoprisms from four samples was carried out. The positions of the main SPRs of these samples were well separated as can be seen in FIG. 19A. TEM grids of samples were prepared such that many of the particles were arranged in a stacked formation with their flat faces parallel to the electron beam. To achieve this it was necessary to concentrate the nanoprisms by centrifugation so that it was possible to measure both their edge-length and thickness. The edge-length measurement has a certain degree of uncertainty as it is possible that some nanoprisms are free to rotate about the <111> axis perpendicular to the flat faces of the nanoprisms, although most are probably resting on an edge in the plane of the TEM grid. The nanoprism measurement data are shown in FIG. 19B and in Table 3, it is clear from this data that there is a distribution of nanoparticle thicknesses within any sample but that the average thickness of nanoprisms from each sample is approximately the same for each sample. The edge-length, on the other hand, displays a clear trend; nanoprisms from each sample have higher average edge-lengths as the spectral position of the main SPR increases.

Examples of TEM images of nanoprisms from samples 1 to 4 are shown in FIG. 27 which demonstrates that the triangular shape of the nanoprisms is established early on in the growth process and that growth proceeds through enlargement of these nanoprisms.

According to theory, the position of the band should depend linearly on edge length and on the inverse of the thickness.^([3]) Indeed, by plotting the λ_(max) against L/T, where L is the nanoprism edge-length and T is the nanoprism thickness, we find a linear relationship as can be seen in FIG. 19C. It is possible to express the edge-length (L) as a function of thickness (T) and the spectral position of the main SPR (λ_(max)) using Equation 1:

$\begin{matrix} {\lambda_{{ma}\; x} = {\left. {{33.8\left\lbrack \frac{L}{T} \right\rbrack} + 418.8}\Rightarrow L \right. = {T\left\lbrack \frac{\lambda_{{ma}\; x} - 418.8}{33.8} \right\rbrack}}} & \left( {{Equation}\mspace{14mu} 1} \right) \end{matrix}$

To study the optical properties of the nanoprisms, a series of samples of increasing edge-length was prepared. These samples display a progression in color as the main SPR is increasingly red-shifted as the nanoprism edge-length increases. These samples are shown in FIG. 20A with the spectra shown in FIG. 20B and replotted against energy in FIG. 20C. Since the spectra were obtained from taking UV-Vis measurements of an ensemble of nanoprisms in solution with a distribution of sizes, all the spectra are inhomogeneously broadened. This means that it is not possible to obtain absolute information about the dephasing time of the plasmons by measuring the line widths (FWHM) of the spectra. However, it is possible to observe how the inhomogeneously broadened line widths vary as a function of plasmon resonance energy and nanoprism volume.

The lateral dimensions of the triangular nanoparticles can be controlled by adjusting the extent of growth. This is controlled by adjusting the number of seeds in the reaction, which in turn is determined by the volume of seed solution used in this growth stage. There is a linear relationship between the position of the in-plane dipole plasmon band and the dimensions of the nanoparticles.

The ultimate size of the nanoprisms can be tuned by controlling the ratio of silver ion: silver seed in the growth step. For example for the samples 1 to 10 described herein the following ratios may be used:

Sample Colour of sample in Mole ratio of silver seed: No. FIG. 20A silver ion Seeds Yellow — 1 Orange 1:9.74  2 Red 1:12.71 3 Pink 1:15.82 4 Purple 1:24.35 5 Royal blue 1:31.65 6 Blue 1:52.82 1 Turquoise 1:70.24 8 Aquamarine  1:105.63 9 Light blue  1:158.23 10 Lilac  1:316.46

Examples of four samples (1 to 4) with TEM analysis are shown in FIG. 27. The series of successively larger nanoprisms were synthesised according to the process described in this example with volumes of seed solution of 650 μl, 300 μl, 150 μl and 130 μl.

In FIG. 21A, the line widths (FWHM) of each of the SPRs from FIG. 20C are plotted against plasmon resonance energy. It can be seen that the width of the main SPR (in-plane dipole) increases as the energy of the resonance increases. This is consistent with measurements of the scattering spectra of individual silver nanoprisms by Munechika et al^([20]) who showed that the line width of the SPR of individual nanoprisms increased with plasmon resonance energy and that this was also correlated with nanoprism volume. Overall, this trend of increasing line width could be explained as due to both increased radiation damping and increased non-radiative decay.

In FIG. 21B, the SPR line widths are plotted against nanoprism volume and it can be seen that in our samples the line widths of the SPRs decrease as nanoprism volume increases (the SPR energy scales inversely with nanoprism edge-length). The TEM studies of FIGS. 17 to 21 show that as the particles become larger, there is no decrease in polydispersity of the samples, i.e. no focusing of the growth conditions to produce a sample with a narrower size distribution. In fact, there is a steady increase in the edge-length distribution as edge-length increases, see Table 3, yet the line widths decrease. This means that the narrowing of the line widths with increasing nanoprism volume must be due to a narrowing of the line widths of the SPRs of the individual silver nanoprisms in the samples with increasing size.

TABLE 3 Sizing data for samples 1 to 4. Edge-length Thickness Sample [nm] σ_(Edge-length) [nm] σ_(Thickness) λ_(max) [nm] 1 20 3 5.1 1.1 538 2 33 7 5.4 0.9 626 3 46 8 5.9 1.2 707 4 65 14 5.6 1.0 796

The degree of radiation damping increases with nanoparticle volume, so the degree of radiation damping must be increasing. However, since the line widths narrow with increasing volume, this indicates that there is an overall decrease in plasmon damping. Therefore there must be a decrease in non-radiative damping that far outweighs any increase in radiation damping. Thus the degree of radiation damping in these nanoprisms must be very small. This behaviour is very similar to that observed in gold nanorods with a narrow range of diameters but with a large range of aspect ratio,^([18]) On the other hand, Munechika et al. show that the line widths of the SPRs obtained from their individual silver nanoprisms increase with nanoprism volume.^([20])

If the in-plane dipole SPR is sufficiently red-shifted and the samples are sufficiently monodisperse, then the in-plane quadrupole SPR should be visible. This is clearly the case for many of the spectra in FIG. 20A. Calculations have shown that as silver nanoprisms get even larger, higher order multipole resonances should become visible^([3,60]) Higher order multipole resonances in nanorods are well documented^([61,62,63]) and have also been observed in silver nanospheres.^([64]) A closer look at some of the spectra for the samples of our largest nanoprisms show a shoulder on the in-plane quadrupole resonance at 465 nm (see FIG. 28). While this could possibly be due to extinction by another species we provisionally assign this as an in-plane octupole resonance.

Optical properties aside, the structural properties of silver nanoprisms are a source of much interest. The TEM analysis of these nanoprisms in this Example provides direct evidence of a defect-induced arrangement of silver atoms that not only results in a hcp structure in the vicinity of the defects but also in multiple defects combining to yield a continuous hcp lamellar region of about 1.5 nm in thickness. As shown in detail below, this hexagonal arrangement of atoms propagates perpendicular to the flat {111} face of the nanoprism with a spacing of 2.50 Å and thereby explains the commonly observed 2.50 Å lattice fringes in flat-lying silver nanoprisms as shown in FIG. 22.

To investigate this possible hcp arrangement of atoms we have performed detailed TEM studies of vertically oriented silver nanoprisms. A typical sample of flat-lying nanoprisms is shown in FIG. 17A. For a defect in the <111> direction to be observed in the TEM, it is necessary that the nanoprism is oriented such that a {110} plane is in the plane of the image. In this orientation, two {111} planes and a {100} plane are aligned vertically with respect to the electron beam. The defects can then be detected as discontinuities in either the {100} or {111} planes that propagate away from the flat face of the nanoprism. This is illustrated schematically in FIG. 23.

For the correct orientation to occur, the nanoprisms firstly need to be vertically orientated as in the stacked formation as shown in FIG. 17B and secondly need to have one edge parallel to the electron beam (see the left hand side of FIG. 23). This means that few nanoprisms will have the {110} plane correctly aligned since most nanoprisms are probably resting on one of their edges on the TEM grid. However, some nanoprisms do have the right orientation and a layered defect structure is visible in two of the stacked silver nanoprisms in FIG. 24A. Closer inspection of the nanoprism on the right reveals that it is indeed being observed along <110> as the internal defect structure of the crystal is visible (FIG. 24B). An analysis of the defects is shown in FIG. 24C. The flat {111} face of the nanoprism is clearly indicated and lattice fringes corresponding to {111} planes can be seen propagating away from the face of the nanoprism, parallel to the {111}-labeled side of the hexagon. The spacing between these fringes was measured to be 2.35±0.05 Å, the correct spacing for {111} planes. Further away from the face of the nanoprism, these {111} planes show discontinuities due to repeated stacking faults between the {111} planes parallel to the face of the nanoprism. There is now an arrangement of atoms that propagates perpendicular to the flat face of the nanoprism, indicated by the two white lines. Significantly, this perpendicular arrangement of atoms has a periodicity of 2.50±0.05 Å, corresponding to the lattice spacing that is observed when a flat-lying nanoprism is observed along <111> (FIG. 22).

In fact, there are so many defects in the nanoprism here that a significant continuous portion of the crystal has a hcp arrangement; a lamellar region about 1.5 nm thick. This is highlighted by the superposition of a zigzag pattern on the TEM image in the top of FIG. 24C. Assigning each apex on this pattern to an atom in alternate A and B layers (atomic planes) of the hcp lattice, the average measured distance between an A and B layer in this region is 2.35 Å, which is the spacing between {111} planes in an fcc lattice, which are stacked in an ABCABC . . . configuration. Since the spacing between alternate layers in an ABABAB . . . configuration is the same as that in an ABCABC . . . configuration, each A and B point on the zigzag pattern therefore corresponds to atoms in alternate A and B layers of a hcp lattice.

The reconstruction of the silver lattice is illustrated schematically in FIG. 23. By introducing a series of intrinsic stacking faults (isf) it is easy to see how these defects give rise to an ABABAB . . . stacking arrangement of the atomic planes in a region of the nanoprism. The perpendicular arrangement of atoms with respect to the flat {111} face of the nanoprism is indicated and has a 2.50 Å spacing.

Further evidence of the transformation of the crystal structure to hcp is provided by x-ray diffraction (XRD) data from our nanoprisms. These show peaks for the fcc silver lattice, as expected, but also show two further peaks corresponding to reflections that are predicted to arise from a hcp arrangement as indicated in reference 57. This is shown in FIG. 25.

Since a significant portion of the nanoprism maintains its fcc structure, as evidenced by the TEM analysis in FIG. 24 and the XRD data, we cannot preclude observation of fcc lattice fringes in <111> oriented flat-lying silver nanoprisms. Indeed, this has proven to be the case with the recent observation of 1.44 Å lattice fringes in silver nanoprisms, arising from {220} reflections;^([65]) and 1.24 Å lattice fringes in silver nanoprisms, arising from {311} reflections.^([38])

The familiar triangular shape and constant thickness of nanoprisms results from highly selective lateral growth from the edges. Due to the lamellar defect structure of the nanoprisms, it is precisely at these edges where the defects are exposed to the growth solution. Thus the significant rearrangement of the crystal structure described here very likely plays a crucial role in giving rise to two-dimensional growth. The hcp crystal faces (or defect-rich regions) at the edges must support a much faster rate for the addition of silver atoms during growth, compared to the {111} or {100} faces. Since the hcp structure is not the natural crystal structure for silver, it must therefore be less stable than the fcc structure, making it likely that the edges where the hcp structure is exposed to the growth medium are less stable than the {111} or {100} faces. This higher degree of instability may be the basis of the faster two-dimensional growth at the edges. The hcp and fcc crystal structures both have a hexagonal symmetry so it remains to be explained why triangles, and not hexagonal nanoplates, are the preferred outcome of two-dimensional growth.

To explain this let's consider a flat, <110> oriented, fcc single crystal as shown in the schematic in FIG. 26A. It is not proposed that a single fcc crystal would take up such an anisotropic structure, but it is clear that it can be cut such that opposite sides could have alternating {111}/{100} pairs of faces. The fcc crystal has six-fold symmetry around the <111> axis so a hexagonal platelet could have the alternating faces as outlined in FIG. 26B, although the relative sizes of each face at an edge would not necessarily be as fixed as the diagram suggests.

Next consider a more realistic version of a hexagonal nanoplate that could be the result of initial two-dimensional growth from the seed, see FIG. 26C. This possesses the hcp (or defect-rich) region sandwiched between two fcc regions, corresponding to what our TEM data suggest. The schematic is drawn such that the regions on either side of the central hcp region are asymmetric. The thickness of each fcc layer would then define the size of each of the respective crystal faces on each edge. This would mean that not all of the edges of the nanoplate are identical; three of them have a larger {100} face than the {111} face while the other three have a larger {111} face than the {100} face. We propose that the three edges with the larger, more stable {111} faces will grow more slowly than the other three, consistent with the lack of growth on the flat {111} face of the nanoprisms. The other three, with the larger, less stable {100} faces, will grow significantly faster, leading to the formation of a triangular nanoprism early on during growth. Thus, the asymmetry in thickness between the fcc layers on either side of the hcp layer defines triangular as opposed to hexagonal growth. After a triangular shape is formed, growth continues on the less-preferred edges with the smaller {100} faces, and as it does so, it opens up the preferred growth edges at the apices of the nanoprism for continued growth. Since these preferred edges always grow faster, the nanoprism maintains its triangular shape, with both types of edges growing in a concerted fashion. In this manner smaller triangular nanoprisms grow continuously into larger triangular nanoprisms without any significant increase in thickness. In cases where there is no asymmetry in thickness between the fcc layers on either side of the hcp layer, hexagonal nanoplates are expected.

At this point it is worth recalling the silver halide growth model,^([53,54,55,56]) where anisotropic growth is promoted by the presence of twin planes in nanoparticle nuclei. At the edges of the nuclei where the twin plane is exposed to the growth medium, alternating concave and convex {111} surfaces are formed with growth occurring much faster on the concave surfaces, which grow themselves out of existence leaving a triangular nanoprism of a size defined by the size of the particle at the time of twinning.^([53,54]) The presence of two twin planes can result in larger hexagonal nanoplates being formed. This silver halide model for anisotropic growth is not entirely consistent with our observations. Firstly, our analysis shows that several stacking faults can be present in a nanoprism and can even combine to yield continuous hcp regions. Secondly, our nanoprisms do not stop growing once the triangular shape has been established. As can be seen in FIG. 27, the triangular shape is established early on in the synthesis and larger nanoprisms can be formed, long after any concave surfaces at the edge would have grown out of existence, leaving only convex {111} surfaces. Yet in our samples, growth continues at the edges without any increase in nanoprism thickness, i.e. no growth on the flat {111} faces of the nanoprisms.

The faster growth on hcp and fcc {100} faces runs counter to what is normally observed in noble metal nanorod and nanocube syntheses that involve the use of surfactants or polymers to influence shape. As mentioned earlier, in these syntheses organic species tend to prefer to stick to less stable crystal faces such as {110} and {100} leading to preferred growth on {111} planes. We note that in our synthesis the amounts of potentially shape-directing organic species are very low. For example, PSSS is used to enhance the quality of the seeds during their synthesis, but even though growth is quite uncontrolled when it is completely absent, a very large fraction of the particles are anisotropic and are mostly nanoprisms and nanoplates. Also, as mentioned earlier, the amount of citrate used in the growth step of our synthesis is very low. It seems plausible therefore that in the absence of strongly coordinating species, the lower stability of {100} faces may lead them to grow faster than {111} faces.

We have devised a straightforward, non-photochemical, room temperature procedure for the synthesis of silver nanoprisms. The as prepared silver nanoprisms are sufficiently monodisperse that it has been possible to investigate trends in the extent and nature of plasmon damping through measuring the inhomogeneously broadened line width of the SPR from UV-Vis measurements. We have found that there is a decrease in plasmon damping with decreasing plasmon resonance energy, consistent with observations by other researchers. However, the decrease in plasmon damping is also associated with an increase in nanoprism size: Radiation damping scales with nanoparticle volume so the observed decrease in overall damping implies that the amount of radiation damping is small and whatever increase in radiation damping there is, it is outweighed by the decrease in non-radiative damping as the plasmon resonance energy decreases. These results indicate that, at least at low plasmon resonance energies, there is very little damping of the plasmons and that thin (˜5 to 6 nm thick) silver nanoprisms are ideally suited for applications that rely on enhancement of the local field. This needs to be confirmed by measurements on individual nanoprisms of the homogeneous line width.

We have shown that silver nanoprisms possess many defects in the <111>direction perpendicular to the flat face of the nanoprisms and that these can combine to give rise to a hcp layer sandwiched between two fcc layers. This hcp layer has a periodicity of 2.50 Å that provides an explanation for the commonly observed 2.50 Å lattice fringes in flat-lying nanoprisms. Furthermore, this two-dimensional hcp layer is most likely the main explanation for the two-dimensional lateral growth, with the triangular shape of the nanoprisms being driven by the asymmetric distribution of crystal faces at the edges, which is in turn determined by the asymmetric thicknesses of the fcc layers on either side of the hcp layer. The silver halide model is perhaps a good starting point for understanding anisotropic growth in as much as it identifies defects as crucial, however it apparently does not adequately explain the growth patterns of metal nanoprisms. We believe that the defect-induced arrangement of silver atoms into continuous hcp regions, as reported here, represents a significant insight into the growth mechanisms of anisotropic metal nanoparticles.

Example 8 Synthesis of Size and Colour Tunable Silver Nanoparticles

In this example we used a generic microfluidic chip system for the production of silver seeds (step (a)) and step b) was carried out by systematically changing the volume of seeds added to the growth step. The nanoparticles produced were size and colour tuned triangular nanoplates (nanoprisms).

Step (a)

Referring to FIGS. 3A and 4, solution 1 comprised 100 ml of 5×M silver nitrate. Solution 2 comprised a mixture of 3 mL of 10 mM sodium borohydride, 2.5 mL of 500 mgL⁻¹ poly(sodiumstyrene sulfonate) and 100 mL of 2.5×10⁻³M trisodium citrate in water.

Solution 1 and solution 2 were connected to pump 1 and pump 2 respectively (see setup of FIGS. 3A and 4). The flow rates of pump 1 and pump 2 were set for example at 1 ml/min and 1 ml/min respectively. The pump lines were primed with the solution to be used in them and pump 1 and pump 2 were run in succession for ˜2 min each such that an initial volume of ˜2 mL of each solution was run through the chip and discarded. Pump 1 and pump 2 were run together and the first 1 ml of the product solution was discarded. The subsequent 5 ml of seed product was collected and both the pumps were stopped.

FIG. 29 shows the UV-visible spectrum of the seeds produced. The flow rates, the flow rate ratios, the reagents and their relative concentrations, volumes and mixing configuration, mixing order and conditions and reagents may be adjusted for each specific type of nanoparticle seeds to be produced.

Microfluidics was used for step (b) (growing the silver seeds synthesized in step (a) into nanoparticles) to produce colour tuned triangular nanoplates from the silver seed synthesised by microfluidics method with a flow rate ratio of 1:1 of solution 1 (AgNO₃) and solution 2 (NaBH₄, PSSS and TSC) described above.

Step (b)

5 mL of water, 75 μl of 10 mM ascorbic acid and μL volumes of the seeds, as specified in Table 4, below were stirred together in a beaker using a magnetic at a rate of 500 rpm. Using pump 1, 3 mL of silver nitrate 5×10⁻⁴ M was added at a rate of 1 mLmin⁻¹. 500 μL 2.5×10⁻²M trisodium citrate was then added to stabilize the particles and the final volume was brought up to 10 mL using water.

TABLE 4 Volume of silver seeds added to produce silver nanoplates of a specific size and colour (spectral location of surface plasmon resonance (SPR) peak Volume of seeds added (μL) Position of SPR (nm) 100 656 200 604 300 558 350 544 400 531 600 500

FIG. 30 shows the UV-visible spectrum of a range of triangular silver nanoplates which are size and colour tuned, produced as described above using the silver seed volumes given in Table 4.

Step (b) may be carried out using the high pressure microfluidics process which would enable the production of large volumes of size and shape controlled triangular silver nanoplates. Within the microfluidics production the flow rates, the flow rate ratios the reagents and their relative concentrations, volumes and mixing configuration, order and conditions may be adjusted for each specific type of nanoparticle seeds to be produced. In addition a polymer maybe added at any of solutions or at any of the preparation stages to further modify the surface chemistry, the stability or the durability of the silver nanoparticles for applications such as functionalisation or industrial processing.

Example 9 Microfluidic Method of Growing Silver Seeds

A microfluidics processor technology as described above in examples 4 and 5 was used to create seven batches of discrete high definition silver nanoparticle solutions of varied formulations, as described in Table 5 below.

TABLE 5 Seven example formulations of silver nanoparticle inks which were produced PSSS Seed Silver Nitrate:Trisodium Formulation (mg) Volume (μl) Citrate ratio 1 20 250 1:1 2 5 500 1:2 3 20 500 1:2 4 100 500 1:2 5 10 500   1:1.5 6 10 500 1:2 7 10 500 1:1

The invention is not limited to the embodiment hereinbefore described, with reference to the accompanying drawings, which may be varied in construction and detail.

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1-100. (canceled)
 101. A process for synthesising silver nanoparticles comprising the steps of: (a.) forming silver seeds from a silver source solution and a reducing agent solution; and (b.) growing the thus formed silver seeds into silver nanoparticles. wherein at least step (b) is performed using pressurised microfluidic flow chemistry.
 102. The process as claimed in claim 101 wherein the silver seeds are grown in step (b) by mixing a silver source salutation and a reducing agent solution.
 103. The process as claimed in claim 102 wherein the silver source solution comprises silver seeds.
 104. The process as claimed in claim 102 wherein the reducing agent solution comprises silver seeds.
 105. The A process as claimed in claim 102 wherein the silver source solution comprises a stabiliser.
 106. The process as claimed in claim 102 wherein the reducing agent solution comprises a stabiliser.
 107. The process as claimed in claim 102 wherein the solutions are pressurised to at least about 35 MPa.
 108. The process as claimed in claim 102 wherein the salutations are pressurised in the range of about 35 MPa to about 275 MPa.
 109. The process as claimed in claim 102 wherein the solutions are pressurised at about 140 MPa.
 110. The process as claimed in claim 107 wherein the solutions have a shear rate of at least about 1×10⁶ s⁻¹.
 111. The process as claimed in claim 107 wherein the solutions have shear rates in the range of about 1×10⁶ s⁻¹ to about 50×10⁶ s⁻¹.
 112. The process as claimed in claim 102 wherein the solutions have a flow rate of at least about 10 ml/min.
 113. The process as claimed in claim 102 wherein the solutions have a flow rate of at least about 100 ml/min.
 114. The process as claimed in claim 102 wherein the solutions have a flow rate of at least about 11/min.
 115. The process as claimed in claim 102 wherein the solutions have a flow rate of at least about 101/min.
 116. The process as claimed in claim 102 wherein the solutions are introduced separately.
 117. The process as claimed in claim 116 wherein each solution has a different flow rate.
 118. The process as claimed in claim 116 wherein each solution has the same flow rate.
 119. The process as claimed in claim 102 wherein the solutions are added at different concentrations.
 120. The process as claimed in claim 102 wherein the residence time of the solutions in a mixing chamber of a microfluidic system is less than about 1 ms.
 121. The process as claimed in claim 102 wherein the residence time of the solutions in a mixing chamber of a microfluidic system is between about 2 μs and about 1 ms.
 122. The process as claimed in claim 101 wherein step (a) is performed using microfluidic flow chemistry.
 123. The process as claimed in claim 122 wherein step (a) is performed using pressurised microfluidic flow chemistry.
 124. The process as claimed in claim 123 wherein the solutions are pressurised to at least about 35 MPa.
 125. A process for synthesising silver nanoparticles comprising the steps of: (a.) mixing a silver source solution and a reducing agent solution to form silver seeds; and (b.) growing the thus formed silver seeds into silver nanoparticles wherein step (a) or (b) is performed using microfluidic flow chemistry and wherein step (a) or (b) is performed in the presence of a water soluble polymer. 